[erlang-questions] Binary match in function head doesn't compile

[Erik Pearson]

Hi,

I'm wondering why this

test(<<Field:Len/binary, Rest/binary>>, Len) ->
    {Len, Field, Rest}.

does not compile, complaining that "variable 'Len' is unbound", while this

test(<<Field:2/binary, Rest/binary>>, Field) ->
    {Field, Rest}.

does. For some reason the compiler doesn't see the Len from the match spec
in the arguments, but it does see Field. Is that by design?

BTW  supplying a variable for Len does work in this case:

test(Bin, Len) ->
  <<Field:Len/binary, Rest/binary>> = Bin,
  {Len, Field, Rest}.

Any insights?

The reason I ask is that a way of saving memory allocation when parsing
binaries might be to walk the binary recursively, incrementing the Len,
while matching on something else that follows. This would result in a
sub-binary reference and not a sub-binary copy. This is not my idea (thanks
for the tip Dimitry) but I think it loses efficiency if the match state
can't be carried recursively in the function head.

Thanks,
Erik.
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